There is known a glowing cathode (cf., Schats M.F. "Heaterless Ignition of Inert Gas. Ion Thruster Hollow Cathodes" AJAA Paper, 1985) comprising a casing, a cylindrical insert secured to the inner surface of the casing and functioning as thermal emitter, a heater secured at the outer side of the casing, and an orifice secured to end face of the casing and acting as the outlet hole of the cathode. This construction of cathode requires high power heaters to heat thermal emitter to a temperature ensuring thermoionic emission sufficient for maintaining a stable discharge.
There is also known a plasma compensation cathode (cf., L.A. Artsimovich, et al. "Razrabotka statsionarnogo plazmennogo dvigatelya i ego ispytanie na iskusstvennom sputnike Zemli Meteor", Kosmicheskie issledovania, 1974, tom XII, vyp. 3, pages 455 and 456, FIG. 5). This compensation cathode has a casing with an outlet hole at one wall thereof, the casing accommodating coaxially to its outlet hole a tubular holder receiving a thermal emitter with a central through passage. The compensation cathode also includes a heater embracing the tubular holder, and heat screens positioned between the holder and casing walls. Connected to the tubular holder is a pipe for feeding gas to the interior of the casing and to the passage of thermal emitter through its inlet portion. This pipe is secured in the casing through an insulator.
During operation of the compensation cathode, gas is conveyed through the tubular holder to the passage of the thermal emitter. Heated to a high temperature, the thermal emitter ensures emission of electrons sufficient for maintaining stable electric discharge between the inner surface of the thermal emitter and anode of the plasma source. After bringing the device to steady-state operation conditions the heater is deenergized, and the compensation cathode continues to operate automatically, whereby the preferred temperature level is ensured by the energy liberated in the catholyte layer approximating to the product of ionic current resulting from discharge by the potential drop at the cathode. However, in the course of operation the discharge can move from the passage of thermal emitter to the interior of tubular holder resulting in evaporation of the material of the holder and fouling of the passage with holder material to almost complete clogging. As a result, thermoemission surfaces tend to degrade, and thermoemission current tends to decrease thereby reducing the service life of the compensation cathode to only tens of hours. In addition, direct connection of the holder of thermal emitter to the gas feeding pipe leads to vigorous heat transfer from the emitter to outer structural parts, and consequently to move prominent catholyte potential drop ensuring the energy necessary for maintaining automatic operating conditions. More prominent catholyte potential drop also leads to reduced service life of the thermal emitter because of intensified ionic bombardment. In addition, tight contact of thermoemissive materials with the holder at high working temperatures is accompanied by active chemical interaction, such as penetration of boron followed by formation of metal borides, which in turn causes embrittlement and cracking of the holder material and thermal emitter to result in irreversible deformation of the holder. This disadvantageous effect is especially pronounced at starting operating conditions accompanied by the highest temperature levels, which limits the service life and reduces the total number of engagements of the compensation cathode. Also, the helical heater embracing the tubular holder is characterized by low rigidity to result in sagging and deformation of its coils resulting in possible contact of the coils with the holder or thermal screens and short-circuiting of the heater. This in turn leads to fewer engagements of the compensation cathode and reduced service life thereof. In addition, the working gas can contain negligible quantities of such admixtures as oxygen, water, or the like, tending to react at high working temperatures with the material of the thermal emitter and affecting the thermoemissive characteristics of the material. Extended operation for tens or hundreds of hours makes this disadvantageous effect even more prominent to reduce the service life of the compensation cathode.